In recent years, increasing demand for higher speed and increase in the capacity of the network has lead to an increase in the importance of digital coherent optical communication techniques. A digital coherent receiving scheme is also referred to as an intradyne reception scheme. The digital coherent receiving scheme can achieve improvement in the receiver sensitivity of 3 to 6 dB or more than the modulation schemes used in large-capacity optical communication systems such as OOK (on-off keying) and DPSK (differential quadrature phase shift keying). Moreover, the digital coherent optical communication scheme has features such that the digital coherent optical communication scheme has a high affinity for multilevel modulation schemes including a polarization multiplexing scheme and QAM (quadrature amplitude modulation).
FIG. 10 is a diagram showing a configuration of a digital coherent receiver disclosed in patent literature 1 and non patent literature 1. An input optical signal is a four-channel (Ix, Qx, Iy, and Qy) multiplexed signal using a DP (dual-polarization)-QPSK (quadrature phase shift keying) signal. The input optical signal is input to a polarization diversity 90° hybrid 1001 and converted by an Optical/Electrical (O/E) converter 1002 into analog electrical signals by different channels. Then, the O/E converted signals are converted into digital signals by A/D (analog to digital) converters 1003 that perform sampling at timings synchronized with a reference sampling clock (CLK) 1004.
The signals converted into the digital signals by the A/D converters 1003 are input to a digital signal processing circuit 1005. Below is an explanation for a background of using a digital signal processing circuit in the digital coherent receiving scheme and a function of the digital signal processing circuit.
There has been an issue in the coherent receiver not associated with the digital signal processing that stable reception is difficult due to an offset and polarization fluctuation in frequency and phase of LO (local oscillator) light.
Meanwhile, development of the electronic device technology enabled high-speed A/D converters to be used in signal processing of high-speed communication devices. Consequently, the digital signal processing performed on converted digital signals has allowed compensation of the offset in the frequency and phase of LO light, which has been the issue in the coherent receiving scheme not associated with the digital signal processing. The digital signal processing has further enabled compensation of polarization fluctuation in optical signals.
As has been mentioned, the digital coherent receiving scheme is capable of more stable and accurate coherent reception than the coherent receiving schemes not associated with the digital signal processing. In addition to the abovementioned compensation of the frequency and phase offset and compensation of polarization fluctuation, the digital coherent receiving scheme further makes possible compensation of wavelength dispersion and higher-level waveform equalization technique.
Non patent literature 2 describes a skew compensation technique as a technique to compensate waveform distortion in a digital coherent receiver. Non patent literature 2 describes a technique to realize highly accurate skew compensation by quadratic function approximation from adjacent, previous, and next sampling points using an FIR (finite impulse response) filter.
Moreover, the spread of high-speed A/D converters has enabled backplane transmission, for example, that conducts inter-substrate connection to carry out higher-level waveform equalization and highly accurate digital clock extraction using the digital signal processing such as MLSE (most likelihood sequence estimation) to address issues such as intersymbol interference and deterioration in jitter characteristics caused by insufficient bandwidth of transmission paths.